This invention relates to the field of phased array antennas, and, more particularly, to a method and apparatus for antenna beamforming.
Phased array antenna systems are widely used in radar, electronic warfare and high data-rate communications applications. A portion of a conventional multibeam phased array antenna system 20 is shown in FIG. 1. The antenna system includes a plurality of radiators 22 that are arranged along an array face 24. The radiator array is typically divided into subarrays. For example, the array might contain 1024 radiators that are divided into four subarrays that each contain 256 radiators. The term radiator is used to refer to both the transmitter and receiver aspect of the antenna system. For simplicity, FIG. 1 illustrates a single 16 element row in one of these subarrays. In each row, each radiator 22 is coupled by a power amplifier 28 to a respective multiplexer 30. Each radiated beam is associated with a different manifold 32 that has a primary transmission line 34 which branches into secondary transmission lines 36 that each couple to a respective one of the multiplexers 30. A programmable delay line 38 is inserted into the primary transmission line 34 and a filter 40 and an adjustable electrical phase shifter 42 are inserted into each secondary transmission line 36. For clarity of illustration, each primary transmission line is labeled with the number of its respective antenna beam.
Operation of the phased array antenna can be separated into coarse and fine beam pointing processes. In a coarse beam pointing process, an appropriate time delay is programmed into each beam #1 delay line of the four subarrays. These time delays generate a selected coarse phase front (e.g., the coarse phase front 44) across the antenna array and, accordingly, a #1 antenna beam is radiated orthogonally to that coarse phase front. In a fine beam pointing process, appropriate phase shifts are selected with the phase shifters 42 that are associated with the manifold of beam #1. These phase shifts modify the coarse phase front to generate a fine phase front (e.g., the fine phase front 46) across the antenna array and, accordingly, the #1 antenna beam is radiated orthogonally to that phase front. This operational process is repeated for each of the other beams, i.e., beams #2, #3 and #4.
However, when data (e.g., pulses) are placed on the radiated signals, the signal spectrum is widened. This can lead to an undesirable increase in beam divergence. This undesirable beam broadening in wide bandwidth signals is commonly referred to as xe2x80x9cbeam squintxe2x80x9d. In the antenna 20 of FIG. 1, the delay lines 38 insert an appropriate time delay to form the coarse wavefront 44. Each radiated beam is preferably coarsely steered to a nominal beam angle and then finely steered about this nominal angle. The coarse steering will not induce beam squint but the fine steering will. It can be appreciated, therefore, that it would be advantageous to have phased array structures that generate antenna beams that have low values of beam squint.
One approach which provides for a wideband phased array antenna system that has less beam squint than conventional antennas is set forth in U.S. Pat. No. 5,861,845, entitled xe2x80x9cWideband Phased Array Antennas and Methodsxe2x80x9d (hereinafter the ""845 patent), which is incorporated herein by reference. Such antennas have no beam squint at the selectable scan angles. Although beam squint increases as the scan angle is varied in response to the frequency of the scanning signal, this increase is controlled by increasing the number of reference differential time delays. In contrast to conventional phased-array antennas, antennas of the type set forth in the ""845 patent have significantly reduced packaging complexity at the array face and are considered an improvement over conventional phased array antennas.
In reviewing the ""845 antenna system in more detail, the antenna system includes an electronic signal generator, reference and scanning manifolds and an array of radiative modules. In transmit mode, the signal generator generates a variable-frequency scanning signal and a reference signal wherein the frequency of the reference signal is substantially a selected one of the sum and the difference of the frequencies of the scanning signal and an operating signal. A reference manifold receives and divides the reference signal into reference signal samples which are progressively time delayed by a selectable one of reference differential time delays. A scanning manifold receives and divides the scanning signal into scanning signal samples which are progressively time delayed by a scanning differential time delay. Each of the radiative modules includes a mixing device, an electromagnetic radiator and a filter. The mixing device receives and mixes a respective one of the reference signal samples and a respective one of the scanning signal samples. The filter couples the mixing device to the radiator and is configured to pass the operating signal. Accordingly, an antenna beam is radiated from the array at selectable scan angles with each of the scan angles varying in response to the frequency of the scanning signal.
In receive mode, operational signals received by the radiators enter mixers and are converted to reference signals with scanning signals that are generated by optical detectors. The converted reference signals are then placed on optical carrier signals in optical signal generators and sent through programmable delay lines. The delayed signals are then detected in optical detectors and combined in a corporate feed to produce a coherent vector sum at a feed output. When receiving incoming operational signals, the delay lines are also programmed as in the transmit operation of the reference manifold. However, in contrast, they are programmed to form conjugate manifolds (e.g., if the manifolds are programmed to generate a transmit beam having a transmit beam angle, they are subsequently programmed to form a receive manifold having a receive beam angle that is the conjugate of the transmit beam angle).
Referring to FIG. 2, a receiver implementation of the invention of the ""845 patent is shown. The scanning manifold described in the ""845 patent generates the local oscillator wavefront Ss. This wavefront is photodetected line-for-line, amplified, then electrically mixed line-for-line with incoming wavefront SO by subsystem 50 (located at the antenna backplane) to produce an IF wavefront which has a frequency Sr. In line switched programmable delay lines 52 then tilt the Sr wavefront to perpendicular propagation 54 and the beam is photodetected and electrically vector summed. The delay lines, photodiodes, and corporate feed correspond to the reference manifold of shown in FIG. 4E of the ""845 patent. It should be noted that for this one dimensional (1-D) design, the signal path for the input beam at S0 to the output at Sr undergoes a single electrical to optical to electrical (EOE) conversion. The system of FIG. 2 can be defined as a scan engine and be represented as shown in FIG. 3.
Referring to FIG. 4, a two dimensional (2-D) receiver beamformer design utilizing the teaching of the ""845 patent can be accomplished by stacking the FIG. 3 scan engines in orthogonal planes. Each row of the antenna array is vector summed by a scan engine, then the row outputs are vector summed by a single scan engine in the vertical (column) direction. As such, now two EOE conversions are required in the signal path and numerous components are needed at the antenna backplane.
While the phased array antenna system as set forth in the ""845 patent provides for a wideband phased array antenna system that has less beam squint than conventional antennas, there still exists, however, a need for not only a wideband phased array antenna system that has less beam squint than conventional antennas, but also one that employs a receiving system that has a less cumbersome implementation, needs minimal EOE conversion steps, and minimizes beamforming components needed at the antenna platform. The present invention as described hereinbelow provides such an antenna system.
In accordance with the present invention, an incoming electrical wavefront is received by an antenna. Laser light is amplitude modulated to provide a synthesized optical wavefront beam. The synthesized optical wavefront is mixed with the incoming electrical wavefront by optical modulation to provide a resultant optical waveform tilted to a coarse scan angle. The resultant optical waveform is transmitted to a predetermined delay line to provide an electrical output from the predetermined delay line corresponding to a main lobe of the resultant optical waveform.
In another aspect of the invention, a method of multi-beam, multi-port phased array antenna beamforming is provided. An incoming electrical wavefront is received by an antenna. A plurality of laser light is amplitude modulated to provide a plurality of synthesized optical wavefront beams. The plurality of synthesized optical wavefronts is mixed with the incoming electrical wavefront by optical modulation to provide a plurality of resultant optical waveforms tilted to respective coarse scan angles. The plurality of resultant optical waveforms are transmitted to predetermined delay lines to provide electrical outputs from the predetermined delay lines corresponding to a main lobe of a respective one of the plurality of resultant optical waveforms.
In a further aspect of the invention, a method of multi-beam, multi-port phased array antenna beamforming involving variable frequency is provided. An incoming electrical wavefront is received by an antenna. A plurality of laser light is variable frequency amplitude modulated to provide a plurality of variable frequency synthesized optical wavefront beams. The plurality of variable frequency synthesized optical wavefronts is mixed with the incoming electrical wavefront by optical modulation to provide a plurality of resultant optical waveforms tilted to respective coarse scan angles. The plurality of resultant optical waveforms is transmitted to predetermined delay lines to provide electrical outputs from the predetermined delay lines corresponding to a main lobe of a respective one of the plurality of resultant optical waveforms.
More particularly, in receive mode, the present invention synthesizes a 2-D phase wavefront which is carried to the antenna elements by amplitude modulated laser light within optical fibers. The synthesized wavefront is then mixed with the incoming wavefront by means of optical modulators located at each antenna element. The mixing process results in a fine phase scan which tilts the resultant wavefront to a coarse scan angle. Wavelength division multiplexing (WDM) is used to select the proper delay lines for final summing of the signals at a photodetector or photodetector array. Multiple beam operations also are made possible by WDM, so that both delay line selection and multiple beam separation at the photodetectors is accomplished simply by switching laser wavelengths.